Musical instruments employing a vibrating mechanical element such as a string to produce sound have been provided heretofore with transducers commonly referred to as "pickups" for detecting the motion of the vibrating element and producing an electronic signal representing this vibration. This pickup signal may be amplified and converted to sound by a loudspeaker. The sound produced from the pickup signal supplements or replaces the sound produced by acoustical interaction of the string, the instrument body and the air. Typically, the instrument body has little or no acoustic response, so that the sound produced from the pickup signal constitutes essentially the entire sound of the instrument. This is the case in the common electric guitar, electric bass and the like.
The sound produced by instruments of this nature dies out progressively after the string is excited. This is particularly so in the case of instruments having little or no independent acoustic response. The sound can be prolonged somewhat by operating the amplification and loudspeaker system at extremely high power levels so that strong acoustic waves representing the original vibration impinge upon the string. Such "acoustic feedback" tends to sustain the vibration of the string, thereby prolonging the note. However, this approach is effective only when the sound produced by the amplification and loudspeaker system is extraordinarily loud. Moreover, the acoustic feedback effect depends upon the acoustic properties of the environment. Therefore, this effect will produce different results in different concert halls.
Various attempts have been made to provide a "sustainer" or device capable of prolonging the notes independently of acoustic feedback from the environment. U.S. Pat. No. 4,245,540 discloses a sustainer incorporating a loudspeaker mounted in close proximity to the strings. The amplified signal from the pickup is passed to the loudspeaker, so that acoustic vibrations produced by this loudspeaker will impinge directly upon the strings. U.S. Pat. No. 4,697,491 discloses a sustainer for a stringed instrument such as a guitar having a body and a neck projecting from the body. An electromechanical transducer is mounted to the neck, remote from the body. The pickup signal is passed to this electromechanical transducer. The transducer vibrates the neck and these vibrations are fed back into the strings. U.S. Pat. No. 3,813,473 discloses an instrument having a "bridge" or string support linked to an electromagnet. An electronic signal derived from the pickup signal is applied to this electromagnet, so as to vibrate the bridge and, hence vibrate the strings. U.S. Pat. No. 4,484,508 describes a generally similar sustainer having an electromechanical transducer adapted to shake the instrument body responsive to the pickup signal, and also having a circuit for progressively reducing the amplitude of the signal so as to provide a controlled fadeout. The fadeout circuit is arranged to provide a quicker fadeout for higher frequency signals.
U.S. Pat. Nos. 4,137,811 and 4,181,058 provide a sustain action utilizing magnetic interaction between a static magnetic field and electrical currents passing through the strings themselves. Thus, a magnet is mounted adjacent the strings, and both the strings and frets of the instrument are electrically conductive. Circuitry is provided for directing an alternating current feedback signal representing the pickup signal through the strings via the frets. The alternating current in each string interacts with the static magnetic field to produce an alternating magneto-motive drive force on the string. U.S. Pat. No. 4,236,433 discloses a sustainer employing an electromagnetically actuated tensioning device for each string, each such tensioning device being connected to a feedback circuit. The signal from a pickup associated with each string is applied through the feedback circuit to the tensioning device, so that the tensioning device will periodically stretch and release the string. The '433 patent also discloses an alternative arrangement wherein an electromagnet or "driver" is juxtaposed with each string so that flux from the electromagnet will impinge directly upon the string. Each such electromagnet is provided with a drive signal representing the signal from a pickup associated with the same string. Thus, variations in magnetic flux of the electromagnet will cause variations in the flux impinging upon the strings. This varying flux tends to excite the string in vibration, provided the string itself is ferromagnetic. U.S. Pat. No. 4,075,921 discloses a generally similar approach, employing a magnetic pickup and a magnetic driver arranged to directly excite a ferromagnetic string. The sustainer may be a hand held, battery-powered device incorporating both a pickup and a driver, and arranged so that the pickup and driver can be aligned with one string of the instrument. Alternately, the sustainer may be built into the instrument and may be provided with separate pickups and drivers for the various strings. U.S. Pat. No. 3,742,113 likewise employs a magnetic pickup and magnetic driver directly associated with each string, with a feedback and amplification circuit connected between the pickup and the driver. The ' 113 patent emphasizes that the feedback circuit or amplifier should have "zero phase shift" so as to provide a driving force "in phase with the string's fundamental frequency of oscillation as transduced by the pickup" so as to reinforce the fundamental mode vibration of the string.
The aforementioned '921, '433 and '113 patents utilize pickups and drivers having a separate ferromagnetic pole piece disposed beneath each string, so as to provide a substantially concentrated magnetic field from each pole piece at normal, undistorted position of the associated string. Separate coils may be provided for each pole piece. U.S. Pat. Nos. 4,580,481 and 4,535,668 disclose a pickup having a unitary, oblong coil and ferromagnetic core extending alterally across the string array. Movable permanent magnets are also provided. By repositioning the permanent magnets, the field direction can be varied so as to provide different phase relationships among the signals induced in the coil by the various strings. U.S. Pat. No. 3,983,777 suggests a pickup having a uniform magnetic field strength across the lateral extent of the string array to suppress variations in pickup response caused by lateral movement of the strings. Other unitary pickups having a single coil and a single ferromagnetic pole piece extending across the string array are shown in U.S. Pat. Nos. 4,364,295 and 4,151,776.
Despite the extensive efforts of the art heretofore, there have been substantial, unmet needs for further improvement. The sustainers available heretofore generally have been inefficient, in that they require substantial electrical power to the drive coil in order to produce an appreciable sustain effect. This high power consumption poses a significant problem where the sustainer draws its power from a battery mounted on the instrument.
Moreover, application of high power to an electromagnetic drive coil in a sustainer tends to produce substantial electromagnetic emissions. Electromagnetic fields radiated from the drive coils impinge upon the pickup and induce unwanted signals. Although the pickups used in electronic musical instruments typically incorporate features for suppressing the effect of stray electromagnetic radiation, these measures are not always perfectly effective. Radiation from the driver can be suppressed to some degree by shielding, but such shielding adds weight, bulk and cost. Thus, there has been a substantial need heretofore for an efficient sustainer capable of providing a powerful sustaining effect with only a modest power input to the driver. There has been a further need for a sustainer which would permit the musician to adjust the action of the sustainer to provide varied artistic effects.